Process for stabilizing cellulose esters



Patented Oct. 10, 1944 2,360,239 rice PROCESS FQR STABILIZING CELLULOSE ES'EEBS Guy A. Kirton, Kingsport, Tenn, and James Battaile Kemp, Dahlgren, Va, assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application January 14, 1942, Serial No. 426,788

6 Glaims.

This invention relates to the removal of small concentrations of volatile acids after the preliminary spray or countercurrent washing of cellulose esters and to the stabilizing of cellulose esters by treating the esters with a dilute solution of a weakly alkaline neutralizing agent, dewatering the mass and then heating to its vapor temperature, followed by the condensing of steam thereon and heating to further drive off any weak volatile acid which may be present.

The commonest cause of instability in the lower fawy acid esters of cellulose is combined sulfuric acid remaining from an esterification repropionyl or butyryl rather than acetyl predominates. With increased ratios of the higher acyl to aoetyl. the products obtained tend to become more impervious and more hydrophobic as cellulose esters of this nature offer considerable resistance to the washing out of the free organic acids or neutralizing of the combined sulfuric acid which remains therein. Therefore, the stabilizing of the hydrophobic cellulose esters has created a problem which was not met with in the 7 case of cellulose acetate.

The procedure which is generally used for processing a. cellulose ester after completion of the initial esterlfication reaction is as follows:

1. Hydrolysis, in dope" solution.

2. Precipitation, separating the ester from the solvent in which it is dissolved.

3. Spray-wahing or counter-current washing of the ester to recover economically the bulk of the organic acids retained in the wet precipitated ester, such washing usually reducing the acidity to approximately 0.1% acid.

4. Washing with successive soakings and draining-S in fresh water until the ester is acid-free, usually 10-30 times.

5. Boiling before, during, or after step #4 to stabilize the ester, such boiling being carried out for 1-24 hours, in some cases with changes of water one or more times during the boiling operation.

6. Drying.

An object of this invention is to provide a method of eliminating the extensive washing step and at the same time stabilizing organic acid mters of cellulose the acyl of which is predominately fatty acid groups of 2-4 carbon atoms,

particularly when those esters have a propionyl or butyryl content of at least 25%. Another object of this invention is to provide a method of refining and stabilizing those esters when they are in a. physical form which is unfavorable to satisfactory stabilizing treatment. A further object of this invention is to provide a more economical means of processing, after the usual counter-current wash, to remove free organic acids, When the cost of wash water is high, such as when using specially processed, color-free water to obtain a low color cellulose ester product, our process becomes applicable with considerable savings in processing costs and time. Other objects of this invention will appear herein.

We have found that lower fatty acid esters of cellulose are readily stabilized by soaking the esters in dilute aqueous solutions of weakly alkaline neutralizing agents such as the alkali salts of the weak volatile acids followed by the rapid removal of the free volatile acids formed thereby. This removal is accomplished readily and quickly by controlled heating of the cellulose esteito or slightly above the boiling point of the weak acid which is formed. In this manner a product is obtained which is free of organic acids and of unstable sulfo-esters or combined sulfuric acid in any form. We have found that by steam treating cellulose esters in accordance with this invention, the time required for the final processing and drying of the cellulose ester is considerably reduced and the resulting product is more uniform than it is possible to obtain in using other treatment methods, particularly with celluose esters having a physical form which resist refining and stabilizing,

We have found that such a treatment can be carried out successfully on partially washed cellulose esters, our process being applied immediately after the precipitated ester has been spraywashed or counter-current washed to remove the bulk of the acid (when such washing is necessary for reasons of economy). In this manner, we accomplish both a stabilization and a reduction in processing costs and operation since by our process the customary extensive washing usually performed after pray-washing is eliminated. However, it is to be emphasized that our process; is not liimted in its application to cellulose esters at this particular stage of the customary processing but may be applied even after the usual extensive washing process ha been applied to remove all the free acid from the product. We have found, however, that with the combination of refining steps, as described herein, the usual extensive washing process can be dispensed with in favor of a more simple washing treatment.

This invention, in its broadest aspects, comprises the steps of,

1. Treating the cellulose ester with a water solution of a suitable neutralizing agent, such as v herein described, the' solution having a concentration usually between 0.01 and 0.5%, this concentration depending roughly on the combined 7 temperature of the liquid, components of the mass but well below that at which decomposition of the cellulose ester i likely to occur. 3

4. Condensing steam onto the cellulose ester I o 15 in this invention. As the carbonates and aceand, 5. Then heating the mass again to the vapor.

temperature of the mass'followed by washing if necessary, and drying. v

sometimesheretofore have been In carrying out our invention, it is preferred K o v o sodium, magnesium and ,calcium acetates and that the cellulose ester be treated with a neutralizing agent to neutralize free s'ulfuricacid in the reaction mixture before it is precipitated so that the only sulfuric material with which the applicants will have to deal is that which is present in combined form. After the catalyst has.

been neutralized, the cellulose ester should then be precipitated and counter-current washed to approximately 0.1% free acid. It is to the cellulose esters obtained after a washing treatment removingall except a low concentration (0.1% approximately) of free organic acids but leaving the combined sulfuric acid that this invention is applicable. The thoroughly washed cellulose, ester is first soaked with anaqueous solution of a salt,such as aqueous sodium acetatef-follpwed b removal of allof the liquid possible by was--.

watering operation. The applicants have found stepafterv the steaming operation: but before drying... By. thisinvention low color cellulose esters are obtained in addition to being exceptionally stable. r

I Although the stabilizing treatment of this invention is adapted for stabilizing lower fatty acid esters of cellulose generally, this process has been found totgive stable products with the hy- ..drophobic esters which ordinarily ar difllcult or impossible to stabilize.

The alkaline salts of weak acids, which acids I have a sufl lciently low boiling point that when formed they may be readily driven oil, are used -tates are the most common and most representative compounds of this nature, it is salts of this type which will be employed to the greatest extent. 'Some.salts, suitable in this connection, are

sodium, magnesiumand calcium carbonates and sodium bicarbonate. .We have also found that weak bases, such as calcium or magnesium hydroxide, may be employed in this connectionto give a satisfactorystabilizedproduct.

that draining andcentrifuging; works satisfactorily for this purpose. The cellulose estermass should be agitated. in the next treatment anda satisfactory method of agitating has been a slow tumbling action. Forinstance, the material may be put through a horizontal heating tube having.

After the cellulose ester from an esteriflcation process, while still present in the reaction mixture, is subjected to neutralization of the free catalyst present and is precipitated and washed.

.go-the latter -being preferably a counter-current washing, it is soaked with an aqueous solution of the weak alkali or the alkaline salt of a weak volatile acid. Theconcentration of this alkaline solution is. ordinarily between .01 and 0.5% and ,is roughly governed by the amount of combined sulfuric acid which is present in the cellulose ester being treated, this in turn being a function of the sulfuric acid concentration during hydrolysis, or, .if unhydrolyzed, during the reaction. I

Some of the neutralizing salts mentioned are of some kind of agitating-equipment, such 8.518.

tralizing agents. For instance, the normal calspiral agitator. The temperature isbrought to,

or slightly above, the boiling point of the weak. volatile acidto drive off such residual acid as may be present. In this step it is also, desirable to pass steam over orthrough the mass which acts as a flushing agent for re moving the acid vapors.

from the system. M I .After this treatment, the ester is treated with some other neutralizing stem, the mass being held at a temperature-be--x low 100 C. so as to causethe condensation oilthe steam on the cellulose ester. This condensation treatment is for the purpose of driving the salts, which may be present, intothe cellulose Y ester so that thorough penetration is obtained.

contact uniformly throughout the individual particles with sulfuric acid which has been split ofl duringthe initial heating and steaming treatment-as well as with any sulfo-ester groups which are not yet hydrolyzed off.- (The concentration of salt in the original treatment is designed to give a slight excess of this agent.) After this penetration has beenaccomplished, the mass is again heated to a point at which-weak volatile acid, formed by reaction of the salt with the hydrolyzed sulfuric acid, is drivenoi'f without damage to. the cellulose-ester. cellulose ester is then dried first by subjecting it to a current of airin the same equipment. If, however, the amount of salts present in the ester is large, it will be necessary to include a Wfl 1l 8 Ordinarily the limited solubility in water; and, therefore, may either be in the .form of a suspension or if only a small proportion is used may be a solution or will be used auxiliary to other more soluble neucium carbonate is only soluble-to a limited extent in water and if a greater-percentage is necessary than is soluble in water, it preferably should be employed in the formof the acid carbonate or employed therewith.

The percentageof the neutralizing salt employed inthe soaklng treatment is roughly governed by the combined sulfuric acid in the cellulose ester. This in turn is governed by the concentration of the sulfuric acid in the hydrolysis of the cellulose ester. If the cellulose ester, after precipitation and counter-current washing, is

centrifuged, the ester will ordinarily retain aptralizing agent to employ. depending upon the percentage of sulfuric acid in the hydrolysis:

The soaking is carried on for a sufllcient time to thoroughly saturate the cellulose ester with the aqueous alkaline solution. Dewatering is then gent should also be proximately moisture; that being the case, the following table shows the percentage of neuapplied so as to remove any excess liquid on the cellulose ester.

The cellulose ester is then placed in a vessel, wherein it will be agitated, if not already present in such avessel. The use of a slow tumbling action, such as obtained by a spiral agitator in a horizontal tube. is satisfactory in this connection. The tube is provided with a heating means, such as a jacket, and in this treatment the mass is heated up to a temperature between 100 and 155 (2., preferably between 120 and 130 0., and the treatment lasts normally for 3-5 hours. The time is governed by that necessary to bringthe cellulose ester uniformly to the desired temperalose ester mass, which steam acts as a flushing agent to aid in removing volatile acid.

' The next step in the stabilizing process is the condensing of steam on the cellulose ester mass to drive the-neutralizing material into the cellulose ester and thus thoroughly penetrate it. This step may be accomplished in the same apparatus as used in the preceding step with the heat reduced below 100 0., preferably with a jacket temperature of l-95 0. Also, this treatment is more effective if wet steam rather than dry steam is passed into the mass. although dry steam may beemployed if a temperature in the lower part of the rangespecifled is used. The temperature employed in this step depends upon the size of thecharge and of the cooling surface, as it is desirable to bring the charge uniformly below 100 C. in a reasonable time and avoid overcooling any part of it to obtain the most economical conditions of operation. The following table gives approximately the time of the cooling period at the prescribed jacket temp rature:

Jacket temperature. "C

The steam is fed into the unit uniformly over the cooling period and it is preferable to use p the steam can be widely varied without affecting the results.

After the condensation period, the temperature is again elevated to obtain a vapor pressure of the volatile acid equal to atmospheric. If an hydroxide was employed as the neutralizing agent, obviously this step may be omitted. When this vapor temperature is attained, the flow of steam may then be cut off or the treatment may be continued for the time desired by the indi= vidual operator.

If a large amount of neutralizing salt was used, it is preferable to subject the cellulose ester to a washing operation at this stage of the process. Ordinarily, however, this is unnecessary, as the amount of salt present is so small its presence in the cellulose ester is not objectionable. The cellulose ester may h n be dried. such as by subjecting it to a current of air.

Although the above steps are best performed at atmospheric pressure, the use of other pressures is permissible. For instance. if a lower vapor temperature is desired, a reduced pressure can be employed and vice versa. With the less stable esters which tend to color at higher temperatures, a lower temperature treatment for a longer time is recommended to avoid any. possibilityof discoloration while removing the sulfur from the cellulose ester.

The following examples illustrate this invention. These examples deal with the stabilizing of a high butyryl cellulose acetate butyrate which was precipitated in powder form, this material being most diflioult to stabilize ordinarily.

Example I A sample of powder precipitated cellulose acetate butyrate, having a butyryl content of 35- 38%, was counter-currently washed in a system in which in the last wash 0.2% sodium acetate was added and the cellulose ester was-soaked in that solution for one hour. The cellulose ester powder (containing 0.1% free organic acid) was then centrifuged and placed in a jacketed tube where it was subjected to a slow tumbling action. The temperature of the material was brought up to 208 F. by jacket heating the tube for minutes while passing a current of steam through the tube over the ester. At the end of 40 minutes, the liquids were completely vaporized, as shown by the increasing temperature of the ester which rose quickly. The jacket temperature was then controlled so that the cellulose ester temperature was approximately 230 F. for minutes. The flow of steam was stopped and a current of air was passed over the ester until dry. The product showed fair stability, freedom from acidity and good clarity.

Example II A sample of a powder precipitated cellulose acetate butyrate, having a hutyryl content of 35-38%, was treated for 24 hours in an aqueous sodium acetate solution of .5% concentration. The mass was then centrifuged and placed in a jacketed tube, as described in the preceding example. The temperature was brought up as before and after two hours, the vapor temperature was reached. The Jacket heat was then reduced at the same time continuing the steam flow and the temperature of the mass dropped back to slightly below the boiling point for hour. The heating was then resumed until the vapor temperature was again attained. The flow of steam through the cylinder was then shut oil and the product was dried in a current of air. Analysis of the product, compared with a regular product of like type, was as follows:

Regular plant Steam treated sample processed sample 4:1 viscosity 35 sec. Acetyl. 13.1%.

y Neutral.

l60 0. heat test. Melted spots. Color (4:1 acetone)" 300 160. Haze (4:1 aeetone) 135. Melting mint 210 0 205 C. Charringpoint 31) C 297 C.

' particularly applicable to va'p'or temperaturel of the liquid components of ave-been precipitated in the mass, followed by lowering the temperature agw i az m' e"; gh i i' rvioug nature, --'-"-'of the mass' to-70-95 f passiiig steainitherethe following example is given illustrate the i --'throu'ghand condensing iti on the cellulose ester. p-r p' i nq fq powdatype'of pr'eipitate: A' 5 3.=' A fmethod of sta'bilizing a' powder-preclplassihaving'ihe follbwtated -cellulose" acetate -butyrate of -35'-*-'38% bui i tyryl which comprises-treating a thoroughlywashed= powder-precipitated cellulose' ester of -=this-type with'anaqueous-solution of an alkalinemetal salt of acetic. acid having a conoenv 2 tration-of 0.(l1-0.5%-;.then removingliquid'therei 250 .from;agitating-theeellulose"ester while heating n t v =1 J the'wet mass in theliquid container to the-vapor was pump (1 to a vessel:.equipped witha suitable temperature oL-ithe' liquid components a of" the .i-agitaton; :fiOOOwJbs. ofvglacialuacetic; acid was- .;mass', Ifollowed by'lowering-the temperature of adde'diandistirred in-well.. Thevfirstfprecipitatthe" mass.to"l0-'95 C.,-- passing-steam there- ;Jingfliquor; consistingiof' approximately 7000=lbs. through andcondensing it on the'cellulose ester. tof ;40;%, acid figured asacetic, acid, from apre- ..4. A method of stabilizing a powder-precipig i l recipitation,@was' added over a period of tated cellulose 'acetate.butyrate of 35-38% bu- 15-45 minutes with good agitation. This re.- 'tyryl .which comprisesttreating a thoroughlyduces the-concentration of acid in the precipil :WSBhBd' powder-precipitated cellulose ester of -tatortdapp'roximately 6064% andat thisstage this type with an aqueous solution of sodium *the ester has separated as'a granular'gel. The acetate having a concentration of OBI-0.5%, 'preeipitation-is completed by the addition of then removing liquid therefrom, agitating the :sufficient acid"('10-15% weak acetic acid),-to re-- cellulose ester while heating the wet mass in duce' the acid concentration inthe precipitator the liquid container to the vapor temperature -to;40.%. The "precipitatecan now be separated of the liquid componentsof the mass, followed :as time granular material bydecantatio'nor by x-by lowering the temperature of the mass to :draining on a screen. After separationthe pre- I IO-9590., passing-steam therethrough and concipitate'may be spray washed to recover the densing it on the cellulose ester. larger-part of the retained acid, after which it 5. A method of stabilizing a. owder-precipiis treated with dilute salt solution and steamed tated cellulose acetate butyrate 01 -38% buas'described herein. v 'tyryl which C D e ting a thoroughlyw laim; i washed powder-precipitated cellulose ester of --1-. A' method of stabilizing a powder-precipi- 5 this type with an aqueous solution of alkaline .tated cellulose'acetate butyrate of 35-38% bumetal carbonate. having a cencentration of 'tyryl which comprises treating a thoroughly 0.010.5%, then removi q t e o a washed-powder-precipitated cellulose ester of tilting e Ce u ose e e e heating t e Wet this type with an aqueous solution ofthe alkali mass in the liquid container to e vapor temmetal salt of a weak volatile" acid stable to mod- 40 perature of the liquid co nen s of he m 'erate' heat having a'eoncentration of 0.01-0.5%, followed by lowering t e temperature of the mass "then: removing liquid therefrom, agitating the t0 9f C... passing steam therethrough and cellulose ester while heating the wet mass to condensmg it 011 the e h f e esterthe vapor temperature of the liquid components A method of stabllizlng P e -p e p of the mass, followed by lowering the temperatated cenfllose acetate butymte f utiir of' the mass to TO-95 c., passing steam WW1 :Whwh comprises t eating a thoroughl therethrou'gh and condensing it on the cellulose washed cellulose ester of this type with an aqueester; Y I 011$ i n of sodium acetate having a concen- L 2.1 A method of stabilizing a pdwdepprecipb tration of 0. 2%, then removing the liquid theretated cellulose acetate butyrate of 35-38% buegitatmg the cellulose ester While heating tyryfwhich'comprise's treating a thoroughlyt wet mass t0 the vapor temperature of the washed powder-precipitated cellulose ester of llquid mp0nent5 0f the mass, followed by lowthi'stype-,witH-n a Solution of a weakly ering the temperature of the mass to IO- C.,

' alkaline salt of acetic acid having a concentrapassing steam therethrough and condensing it on tion of '0.01-'0.5%,'then removing liquid there- 5 the Hume ester- 7 from, agitating theeellulose ester while heating GUY A, KIRTON,

the et mass" ii the liquid container tothe 7 JAMES BATTAILE KEMP. 

